Recently, in the method for producing fine polymer grains (containing the toners for the electrostatic charge image development for use in developing electrostatic latent images formed by electrophotography and electrostatic recording), there is proposed a method for obtaining the grains by dispersing and suspending a polymer solution (e.g., a mixed solution of toner materials) previously prepared by dissolving the polymer in a solvent in an aqueous medium, and then, by removing the solvent, for instance, by either heating the resulting solvent suspension or reducing the atmospheric pressure under which the solvent suspension is provided (see, for instance, JP-B-Sho61-28688, JP-A-Sho63-25664, JP-A-Hei7-152202, JP-A-Hei9-15902, where the term "JP-B-" signifies "an examined published Japanese patent application" and "JP-A-" signifies "an unexamined published Japanese patent application").
In the methods above, the solvent is removed by drying in the liquid, i.e., the solvent is transferred from the liquid droplet of the polymer component into the aqueous medium, and the solvent in the aqueous medium is gasified at the interface between the suspension and the gas. To effectively promote the gasification of the solvent, it is required that the contact area of the suspension with the gas is increased, that the energy is efficiently provided to the suspension in the form of the heat, and that the gasified solvent is efficiently removed at the evaporating interface.
Furthermore, to maintain the polymer grains to yield a sharp particle size distribution, it is necessary that stress is not applied to the liquid droplets of the polymer component during the stage of removing the solvent; and to keep the uniform shape of the particles, it is further required that no anomalous strain is applied thereto. If a stress is applied to the polymer grains that are still in the form of droplets in the stage before removing the solvent therefrom, the grains easily undergo fission to generate finer grains as to result in a broad particle size distribution. Furthermore, if the droplets of polymer grains are squeezed by an anomalous strain during the process of removing the solvent from the polymer grain droplets at the stage the region of elastic deformation is switched into the region of plastic deformation, the polymer grains maintain their shape as they are to the final stage.
Further, if the rate of exchanging the contact interface between the suspension and the gas should be lower with respect to the rate of removing solvents at the contact interface, a film which suppresses the progressive removal of the solvent is formed at the contact interface between the suspension and the gas. Thus, the exchanging rate should be well comparable to the rate of removing solvents.
Accordingly, to effectively perform the solvent removal from the droplets of polymer component by using the methods above, there is required an increase in the contact area per unit volume of the suspension and the gas, a greater quantity of carrier gas which transports the gasified solvent, and a larger heating area per unit volume of the suspension which is capable of providing a calorific value sufficient to comply with the heat of evaporation. Furthermore, it is also required that the exchanging rate is provided as such that it would sufficiently comply with the rate of removing solvents while minimizing the applied stress and strain.
However, while it is relatively easy in a laboratory based production to satisfy all of the conditions above if a small portion of the polymer component suspension is slowly mixed inside a heated flask in such a manner that a stress would not be applied thereto and the evaporated vapor is removed therefrom, it is practically unfeasible to achieve them in a industrial scale production.
More specifically, in case of using a commercially available flask and a polymerization tank in a laboratory based production, in general, the contact area between the suspension and the gas is, for 10.sup.-4 m.sup.3 of the suspension, about 28 m.sup.2 /m.sup.3 per unit volume of the gas, and for 10.sup.-3 m.sup.3 of the suspension, it is about 11 m.sup.2 /m.sup.3 per unit volume of the gas. In a bench size or a pilot equipment size production, the contact area is about 11 m.sup.2 /m.sup.3 per unit volume of the gas for 10.sup.-2 m.sup.3 of the suspension, and is about 2 m.sup.2 /m.sup.3 per unit volume of the gas for 10.sup.1 m.sup.3 of the suspension. In case of performing the process in an industrial base, however, the contact area is about 0.8 m.sup.2 /.sup.3 per unit volume of the gas for 1 m.sup.3 of the suspension, about 0.4 m.sup.2 /.sup.3 per unit volume of the gas for 10 m.sup.3 of the suspension, and is about 0.2 m.sup.2 /.sup.3 per unit volume of the gas for 80 m.sup.3 of the suspension. It can be seen therefrom that the contact area between the suspension and the gas per unit volume decreases with increasing amount of the suspension to be processed, and the heating area per unit volume similarly decreases. Should other processing conditions, for instance, the temperature, the pressure, etc., be set the same, the time necessary for removing the solvent from the suspension increases, as compared with the case of removing a solvent of 10.sup.-4 m.sup.3, to 6 times for removing a solvent from a suspension of 10.sup.-2 m.sup.3 ; to 14 times, to 35 times, to 70 times, and to 140 times as the volume of the suspension increases to 10.sup.-1 m.sup.3, to 1 m.sup.3, to 10 m.sup.3, and to 80 m.sup.3 respectively.
As an apparatus for increasing the contact area between the solution and the gas, there can be used a commercially available apparatus, for instance, a thin film evaporation apparatus-, a thin film defoaming apparatus, a packed tower, a gas-liquid counter flow contact apparatus, etc. However, even if the apparatus above should be used in removing the solvent, the efficiency lowers due to the decrease in the contact area between the solution and the gas per unit volume of the suspension as compared with the case of a laboratory based production scale if the amount of suspension increases, and also, it is still impossible to avoid the stress and the strain from being applied to the suspension. Furthermore, although there is proposed a method of efficiently increasing the contact area with the gas by spraying the suspension, this method also is not free from the large stress that is applied to the suspension.
In addition to the methods of drying in a liquid as described above, there is proposed a method of directly drying the suspension, and as the apparatus for use in drying the slurry suspension, there can be used a commercially available apparatus such as a spray drier, a flush drier, etc. In this method, it is necessary to dry the water used as the medium. Accordingly, the suspension is generally brought into contact with a gas at a temperature of 100.degree. C. or higher. However, in such a case, there are disadvantages as such that the heat efficiency becomes low due to the increase in size of the apparatus as compared with the amount to be processed, and that the operation cost becomes extremely high. Furthermore, as an apparatus for use in the method other than that of directly drying the suspension, there can be mentioned a commercially available apparatus such as a vibration drier, a floating bed drier, a paddle drier, and a steam tube drier. However, they are, by principle, unfeasible for use in the method above, because these apparatuses are designed for drying cake-like materials.
Concerning another aspect of the present invention, i.e., the method of producing the fine polymer grains containing the toners for the electrostatic charge image development for use in developing electrostatic latent images formed by electrophotography and electrostatic recording, there are several methods known in the art. Such methods include the methods of directly producing fine polymer grains from monomers used as the starting materials, by utilizing polymerization reactions such as a suspension polymerization, an emulsion polymerization, a seed polymerization, or a dispersion polymerization. However, the fine polymer grains that are produced by the polymerization methods above suffer problems, such that there is a difficulty in removing the residual monomers and the surfactants; that there is a difficulty in incorporating an insoluble material such as a colorant, an antistatic controller, a surface lubricant, etc.; that the available type of polymers and the available particle size range of the grains are limited; and that the optimal conditions for preparing the grains need to be studied every time the material composition is changed.
In addition, there are methods of preparing the fine polymer grains by finely dividing a polymer that is prepared beforehand by means of a polymerization reaction. Among them, the method of melt kneading and grinding comprises subjecting a coarsely crushed polymer to grinding using a finely grinding machine such as of a mechanical rotation type or a jet type, followed by classifying the resulting ground product to obtain the fine polymer grains, and is a most frequently used production method for obtaining the toners for use in developing electrostatic images. However, the fine polymer grains that are obtained by this method is heterogeneous, and moreover, the grain size is not uniform. Hence, the fine polymer grains obtained by this method suffers disadvantages as such that, for instance, it requires a classification process to obtain particles with size distribution falling in a narrow range.
There is also known a method for obtaining particles which comprises preparing a polymer solution by dissolution into a solvent, and then mist-spraying the resulting polymer solution. However, the fine polymer grains that are produced by this method are not free from disadvantages as such that a uniform grain size cannot be obtained, and that the apparatus for production becomes too large.
Similarly, there also is known a method of adding a poor solvent into a polymer solution previously prepared by dissolution into a solvent, or by cooling the polymer solution previously prepared by heating and dissolution into a solvent, thereby obtaining the fine polymer grains by precipitation. However, this method is disadvantageous in that it is difficult to control the shape of the resulting grains and that the grain diameter are not uniform.
Further, there is known a method comprising dispersing a heated and molten polymer in a medium heated to a temperature not lower than the melting point of the polymer, and then cooling the resulting dispersion to obtain fine polymer grains (see, for instance, JP-A-Sho50-120632, etc.). However, in case an aqueous medium is used in this method, almost all cases require applying pressure, and in case an oil medium is used, it suffers difficulty in, not only cleaning, but also in controlling the shape of the grains.
Recently, from the viewpoint of various advantages such that it has no residual monomers, that it does not use surfactants and that it thereby does not require the removal thereof, that insoluble materials such as colorants, antistatic controllers, lubricants, etc., can be easily incorporated, that it does not require re-examination of the optimal conditions for granularization even in case the material composition is changed, that a sharp particle size distribution is obtained, and that the aqueous medium can be easily cleaned up, recently proposed is a method of producing the grains as described above, comprising dispersing and suspending, in an aqueous medium, a polymer solution (a mixed solution of the toner material and the like) which is prepared beforehand by dissolution in a solvent and then removing the solvent therefrom by, for instance, heating the system or by reducing the pressure (see, for instance, JP-B-Sho61-28688, JP-A-Sho63-25664, JP-A-Hei7-152202, JP-A-Hei9-15902, etc.).
Even in the production method above regarded as the most favorable one among the conventional methods from the viewpoint of increasing the toner performance and the suitability in production, when scaled up to an industrial size, it also suffers difficulty in the solvent removal process due to the reason described hereinbefore.